A useful method for metal sheet forming is provided in “quickplastic” or “superplastic” (“quick plastic” or “super plastic”) forming approaches (QPF or SPF) in which sheet metal is formed into a complex shape in a single-sided forming tool using gas pressure to provide the forming force to transform the sheet metal into a form. In this regard, metal sheets such as aluminum are heated and then formed into shapes using gas pressure. The technology allows complex shapes to be formed from materials otherwise difficult to form to the complex shape. Details in these methods are presented in commonly assigned U.S. Pat. No. 5,974,847 for a “Superplastic Forming Process” issued to Fredrick I. Saunders, et al., on Nov. 2, 1999, and U.S. Pat. No. 6,253,588 for “Quick Plastic Forming Of Aluminum Alloy Sheet Metal” issued to Moinuddin S. Rashid, et al., on Jul. 3, 2001, which are herein incorporated by reference.
In a number of products, such as energy absorbing structures used in transportation vehicles, it would be desirable to form a metal foam portion attached to a formed sheet metal structure so as to form a lightweight compactable structure. The compactable structure would deform at predetermined stress levels when subjected to an impact. Formation of such structures to date has not been feasible due to the cost of formation of the foam portion which needs to be shaped prior to the attachment to the shaped sheet metal. Furthermore, adherence of the shaped foam portion to the shaped sheet metal requires use of an adhesive. The adhesive provides a bond in the composite (between the foam portion and the metal sheet) which has its own properties, thus establishing at least three discrete domains of properties within the composite. The interfacial properties of the adhesive may significantly complicate the design of structures using these materials.
What is needed is a unified and straightforward approach for providing a complex composite structure of a formed metal sheet with a metal foam substrate. Preferably, the composite structure could be formed at low cost in such a manner that the foam does not require significant processing of the foam in the attachment step. The present invention is directed to fulfilling these needs.